Circulators generally contain two basic parts:
i/ a microwave circuit comprising an arrangement of conductors and ferrite blocks, and PA1 ii/ a magnetic circuit providing a magnetic biasing field applied to the ferrite blocks that act as a non-reciprocal media for propagating radio frequency signals throughout the device. PA1 i) forming an integral conductor arrangement consisting of a plurality of strips extending outwardly from a base portion having an opening therein; PA1 ii) folding the arrangement to define a first compartment to accommodate a ferrite block; and PA1 iii) folding the strips inwardly without a ferrite block being inserted into the first compartment to form an arrangement of spaced apart overlaying crossing strips.
An ideal three port circulator transmits power (as shown diagrammatically in FIG. 1) between any two ports in a forward direction only, i.e. from port 1 to port 2, from port 2 to port 3, from port 3 to port 1. In the reverse direction (from port 1 to port 3, from port 3 to port 2 and from port 2 to port 1) no power can be transmitted (i.e. port 3 is isolated from port 1, port 2 from port 3, and port 1 from port 2).
A circulator can be converted to an isolator by connecting a matched load to one of the ports. For example, if port 3 is terminated with a matched load, a drive signal is applied to input port 1 and an antenna is connected to output port 2, then any power reflected from the antenna is directed to the terminated port 3 and dissipated in the load.
A typical prior art strip line lumped element circulator is shown in FIGS. 2 and 3. Conductors 4 connected to terminal ports are sandwiched between ferrite discs 5 and 6 which in turn are located in the gap between magnets 7 and 8. Permanent magnets 7 and 8 are supposed to magnetise ferrite disks 5 and 6 and provide a dc biasing magnetic field in the ferrite disks 5,6 that is necessary for signal circulation between terminal ports. The direction of circulation is determined by the orientation of the applied dc magnetic field and may be reversed by reversing the polarity of the magnets 7,8.
In such prior devices conductors 4a, 4b, and 4c form a multi-layered construction where individual strips are interwoven and their intersections are insulated during assembly. The conductor ends (9a, 9b, 9c) are connected to terminal ports of the circulator and the other ends (10a, 10b, 10c) are attached to a common ground plane.
The pattern of interwoven conductors 4 may be fabricated in two different ways. One approach is based on interweaving and joining separate insulated strip conductors. The other technique employs the technology of multi-layered metal and dielectric deposition on the surface of a ferrite disk. The former method is time consuming and the resulting conductor assemblies may have inconsistent topology. The latter procedure exploits thin film technology and is typically useful in fabrication of low power microwave integrated devices. Increasing power handling capacity may result in a substantial rise in manufacturing cost. Another problem encountered by both fabricating methods is the quality of the connections between conductor ends (10a, 10b, 10c) and the common ground plane, the inconsistent joints causing increased losses and degradation of overall circulator performance.
Homogeneity of the biasing magnetic field inside the ferrite disks is normally desirable for optimum circulator performance. Non-uniformity of the biasing magnetic field associated with the shape of magnets and ferrite blocks may substantially degrade insertion losses and isolation between the circulator ports. The crucial problem of optimising distribution of the biasing magnetic field has been extensively explored and addressed in numerous publications and patents.
In particular, to generate a uniform magnetic field inside ferrite disks it has been proposed to attach ferrite semi-spheres either side of the ferrite discs (see E. F. Schloemann. "Circulators for Microwave and Millimeter-Wave Integrated Circuits". Proceedings of IEEE, vol. 76, No.2, February 1988, pp 188-200). Semi-spherical ferrite segments surrounding the ferrite disks neutralise the demagnetising effect of the disk-shaped ferrites on distribution of the internal biasing magnetic field. They help to preserve uniformity of the internal magnetic field when the system is exposed to a uniform external magnetic field. However, such an arrangement is bulky and only employs the central part of the magnetic system due to tight requirements of homogeneity in the external magnetic field. Ferrite semi-spherical segments are also expensive to produce and, due to the very poor thermal conductivity of ferrite, they impede heat transfer from the ferrite disks. The latter problem may result in substantial degradation of circulator performance with increasing power and/or varying temperature.
DE 2950632 discloses the use of frustoconical ferrites in a junction circulator. This is said to reduce noise and intermodulations by minimising the effect of irregularities in the biasing magnetic field nearby the edge of the ferrite. This, however, requires special fabrication techniques, thus increasing cost. This also increases the thickness of ferrite used, thus impeding heat transfer.
Further, in prior art circulators the ferrite was considered simply as part of the microwave circuit not affecting the DC magnetic circuit. This often resulted in difficulties of thermal stabilisation and the need for complex temperature controlling devices.